<template>
    <div>
        <ul class="list">
            <li 
                class="item" 
                v-for="item in letters" 
                :key="item"
                :ref="item"
                @touchstart="handleTouchStart"
                @touchmove="handleTouchMove"
                @touchend="handleTouchEnd"
                @click="handleLetterClick"
                >{{item}}</li>
        </ul>
    </div>
</template>

<script>
    export default {
        name:'CityAlphabet',
        props:{
            cities:Object
        },
        computed:{
            letters () {
                const letters = []
                for (let i in this.cities) {
                    letters.push(i)
                }
                return letters
            }
        },
        data () {
            return {
                touchStatus:false,
                startY:0,
                timer:null
            }
        },
        updated () {
            this.startY = this.$refs['A'][0].offsetTop
        },
        methods:{
            handleLetterClick (e) {
                this.$emit('change',e.target.innerText)
            },
            handleTouchStart () {
                this.touchStatus = true
            },
            handleTouchMove (e) {
                if(this.touchStatus){
                    if(this.timer){
                        clearTimeout(this.timer)
                    }
                    this.timer = setTimeout(() => {
                        const touchY = e.touches[0].clientY - 79
                        const index = Math.floor((touchY - this.startY)/20)
                        if(index >= 0 && index < this.letters.length ){
                            this.$emit('change', this.letters[index])
                    }
                    },16)
                }
            },
            handleTouchEnd () {
                this.touchStatus = false
            }
        }
    }
</script>

<style lang="stylus" scoped>
    @import '~styles/varibles.styl'
    .list
        display :flex
        flex-direction:column
        justify-content :center
        position: absolute
        right :0
        top:1.58rem
        bottom :0
        width:.4rem
        .item
            line-height :.4rem
            text-align:center
            color  :$bgColor
</style>